1. Field of the Invention
The present invention relates to a variable delivery fuel supply device which is used for an internal combustion engine for automobiles, in particular, for a cylinder injection type gasoline engine requiring highly pressurized fuel and which is capable of controlling an amount of fuel to be supplied to fuel injection valves.
2. Discussion of Background
The variable delivery fuel supply device used for an internal combustion engine for automobiles comprises a plurality of fuel injection valves for injecting fuel to the respective cylinders of the internal combustion engine, a delivery pipe for supplying fuel to the fuel injection valves, a fuel pump for supplying pressurized fuel to the delivery pipe, a low pressure fuel pump for supplying fuel from a fuel tank to the fuel pump, and control means for controlling an injection timing of fuel; a quantity of fuel to be discharged and a discharge quantity of fuel from the fuel pump.
The fuel pump comprises a cylinder, a plunger which performs a reciprocating movement in the cylinder to suck fuel into a pressurizing chamber in a suction stroke and which supplies forcibly fuel in the pressurizing chamber to the delivery pipe in a discharge stroke, the plunger being driven by a driving cam attached to a cam shaft of the internal combustion engine, and an electromagnetic valve which controls a discharge quantity of the fuel from the pressurizing chamber by relieving fuel pressurized in the pressurizing chamber to a low pressure side at a predetermined timing to thereby control a fuel pressure in the delivery pipe to be a predetermined pressure level.
As the electromagnetic valve disclosed in, for example, JP-A-11-200990, a normally closed electromagnetic valve wherein a valve unit is closed when no control signal is supplied to the electromagnetic valve, is generally used. When a valve-opening signal is supplied from the control means, the electromagnetic valve is opened whereby pressurized fuel in the pressurizing chamber is relieved to a low pressure side. The control means is so adapted as to detect a fuel pressure in the delivery pipe and to supply a valve-opening signal to the electromagnetic valve depending on variations of fuel pressure to open the electromagnetic valve. In this case, the fuel pressure of the delivery pipe 2 increases when the fuel pump 3 performs a discharge stroke. Accordingly, the valve-opening signal is supplied in the way of the discharge stroke, and the width of the signal is determined so that the electromagnetic valve is closed at the moment of the completion of the discharge stroke.
FIG. 4 is a diagram for explaining a working stroke of a fuel pump and an operation timing of an electromagnetic valve in the conventional variable delivery fuel supply device, which is described in, for example, JP-A-11-200990. The piston lift quantity in FIG. 4 corresponds to a movement of a plunger performing a reciprocating movement due to the driving action of a driving cam of an internal combustion engine wherein the stroke from a bottom dead point to a top dead point is a pressurizing stroke, and an amount of fuel corresponding to a movement of the plunger in a pressurizing stroke is forcibly supplied from the pressurizing chamber to the delivery pipe. The stroke from the top dead point to the bottom dead point is a suction stroke during which fuel is fed from the fuel tank to the pressurized chamber.
In FIG. 4, strokes A to D respectively represent a characteristic movement in one cycle of the plunger in the fuel pump.
The stroke of the plunger in the conventional fuel pump and the movement of the conventional electromagnetic valve will be explained with reference to FIG. 4.
The stroke A shows a case that a discharge quantity of fuel from the fuel pump is 50% with respect to a discharge quantity in the whole stroke. Namely, a valve-opening signal is supplied to the electromagnetic valve to open it at a 50% position in a one cycle stroke of the plunger in a discharge stroke, and the valve-opening signal is stopped at the top dead point of the plunger, i.e., at the moment of the completion of the discharge stroke, whereby the electromagnetic valve is closed.
The stroke B shows a case that the valve-opening signal is supplied when a discharge quantity is 75%. The valve-opening signal is stopped with the completion of the discharge stroke in the same manner as in the stroke A.
The stroke C shows an operating condition of the fuel pump in such a case that the fuel pump discharges a large quantity of fuel, but a fuel consumption rate is low. For example, the internal combustion engine is operated at a high revolution but an applied load is light, e.g., an engine brake is used. In such case, a fuel pressure in the delivery pipe is maintained at a predetermined value. A discharge quantity of fuel by the fuel pump is 0% and a relief quantity by the electromagnetic valve is 100%. This means that the valve-opening signal is supplied over the entire period of one stroke of the plunger.
The stroke D shows a case that a discharge quantity of fuel by the fuel pump is small, but a fuel consumption rate is high. For example, the internal combustion engine is operated at a low revolution but under a heavy load, In such case, no valve-opening signal is supplied so that a discharge quantity is made 100%.
Among these strokes, a state of the discharge quantity being 0% or 100% does not frequently occur. Under the condition of normal use, the valve-opening signal is supplied in the way of the pressurizing stroke of the plunger and is stopped when the plunger reaches a top dead point as described before. Since the width of the valve-opening signal is determined dependent on a fuel pressure in the delivery pipe, it also varies depending on a revolution number of the internal combustion engine and a load applicable to the engine. Further, it varies in each stroke of the plunger even when the revolution number and the load are constant. Accordingly, the control means supplies a valve-opening signal by calculating a valve-opening timing and a width of the valve-opening signal for each stroke.
However, in the conventional variable delivery fuel supply device for supplying a valve-opening signal to the electromagnetic valve as described above, there was a problem as follows. The time at which a fuel pressure in the delivery pipe reached a predetermined value was sometimes just before the time point that the plunger passed a top dead point in a case that, for example, the fuel pressure in the delivery pipe was near the predetermined value or a fuel consumption rate was relatively high with respect to a discharge quantity of fuel by the fuel pump, which may occur immediately after the starting of the engine. Since a time between the time that the fuel pressure reached a predetermined value and the time that the plunger passed the top dead point was very short, a valve-opening signal was supplied as a pulse signal having a small width. On the other hand, there was anther problem as follows. Since there was a certain delay in the response characteristic of the electromagnetic valve when it received a valve-opening signal, the electromagnetic valve could not follow a pulse signal of small width, whereby control became inoperable and a fuel pressure in the delivery pipe became an unstable condition. In particular, when the internal combustion engine was operated at a high speed, a time of stroke of the plunger was short, and the width of the valve-opening signal was also small. Accordingly, the response characteristics of the electromagnetic valve was insufficient to increase the revolution speed of the engine to the maximum.
Further, there was another problem that since a control unit had also a scattering of control, the electromagnetic valve sometimes closed during the discharge stroke before the plunger reached the top dead point. In such case, the fuel pump might discharge twice whereby a fuel pressure in the delivery pipe increased beyond a predetermined value. Further, when the engine was operated at high revolution speed but with a low load, e.g., under the condition that an engine brake was used, there was a case that a fuel consumption rate was low in comparison with a discharge quantity of the fuel pump, and a condition that a discharge quantity was 0% continued. In such case, a rate of a current fed to the coil of the electromagnetic valve reaches 100%. Since the coil resistance is determined to be small in order to improve the response characteristics of the electromagnetic valve, there causes the problem of an abnormal rise of coil temperature. In order to suppress the temperature rise, the coil resistance of the electromagnetic valve should be increased. However, this invited a further deterioration of the response characteristics of the valve.
It is an object of the present invention to provide a variable delivery fuel supply device providing a stable control of fuel pressure without suffering the influence of the response characteristics of the electromagnetic valve and a scattering of control and without causing an abnormal increase of coil temperature.
In accordance with the present invention, there is provided a variable delivery fuel supply device which comprises fuel injection valves for injecting fuel to respective cylinders of an internal combustion engine, a delivery pipe for supplying pressurized fuel into the fuel injection valves, a fuel pump which sucks fuel from a fuel intake passage through an intake valve into a pressurizing chamber by a reciprocating movement of a plunger in a cylinder and discharges the pressurized fuel into the delivery pipe through a discharge valve, an electromagnetic valve which is located in a relief passage communicating the pressurizing chamber of the fuel pump with the fuel intake passage and is adapted to control a discharge quantity of the pressurized fuel by relieving the pressurized fuel in the pressurizing chamber into the fuel intake passage at the time of opening the valve, and control means for supplying a valve-opening signal to the electromagnetic valve, wherein the supply of,the valve-opening signal from the control means to the electromagnetic valve is stopped when the plunger reaches a predetermined position which passes through a top dead point, in an intake stroke of the plunger from a top dead point to a bottom dead point.
In the present invention, the supply of the valve-opening signal is stopped at the time when at least a response time of the electromagnetic valve has lapsed after the plunger has passed through the top dead point.